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Friday, November 14, 2014

The goTo object pattern in Node.js code

I've been messing around with Node.js lately. Like everyone using Node.js, I've been wrestling with the fact that it forces programmers to use hand-rolled continuation passing style for all I/O. Of course, I could use something like async.waterfall() to eliminate boilerplate and deeply indented nested callbacks. However, since I am crazy, I am using Closure Compiler to statically typecheck my server code, and I don't like the way async.waterfall() defeats the typechecker.

You can stop reading here if you're perfectly happy with using async for everything, or perhaps if you're one of those people convinced that static typing is a boondoggle.

Anyway, I've been using a "goTo object" for my callbacks instead. The essence of the pattern is as follows:

Define a local variable named goTo whose members are your callbacks.

Asynchronous calls use goTo.functionName as the callback expression (usually the final argument to the asynchronous function).

At the end of the function, outside the goTo object, start the callback chain by calling goTo.start().

Here is an example, loosely patterned after some database code I recently wrote against the pg npm module. In raw nested-callback style, you would write the following:

It is slightly more verbose than the naive nested-callback version. However, the indentation level does not grow linearly in the length of the call chain, so it scales better with the complexity of the operation. If you add a couple more levels to the nested-callback version, you have "Tea-Party Code", whereas the goTo object version stays the same nesting depth.

As in the nested-callback style, error handlers must be written by hand in each callback, which is still verbose and repetitive: the phrase if (err) { goTo.finish(err); return; } occurs repeatedly. On the other hand, you retain the ability to handle errors differently in one callback, as we do here with key conflicts on insertion.

Callbacks in the goTo object can have different types, and they will still be precisely typechecked.

The pattern generalizes easily to branches and loops (not surprising: it's just an encoding of old-school goto).

Data that is initialized during one callback and then used in later callbacks must be declared at top-level as a nullable variable. The top-level variable list can therefore get cluttered. More annoyingly, if your code base heavily uses non-nullable type annotations (Closure's !T), you will have to insert casts or checks when you use the variable, even if you can reason from the control flow that it will be non-null by the time you use it.

Sometimes I omit goTo.start(), and just write its contents at top-level, using the goTo object for callbacks only. This makes the code slightly more compact, but has the downside that the code no longer reads top-down.

There is no hidden control flow. The whole thing is just a coding idiom, not a library, and you don't have to reason about any complex combinator application going on behind the scenes. Therefore, for example, exceptions propagate exactly as you'd expect just from reading the code.

The camelCase identifier goTo is used because goto is a reserved word in JavaScript (reserved for future use; it currently has no semantics).

In some ways, this is better and terser than the goTo object. Most importantly, error handling and operation completion are isolated in one location. Also, blocks in the waterfall are anonymous, so you're not cluttering up your code with extra identifiers.

On the other hand, this has some downsides, which are mostly the flip sides of some properties of goTo objects:

Closure, like most generic type systems, only supports arrays of homogeneous element type (AFAIK Typescript shares this limitation update: fixed in 1.3; see comments). Therefore, the callbacks in async.waterfall()'s first argument must be typed with their least upper bound, function(...[*]), thus losing any useful static typing for the callbacks and their arguments.

Any custom error handling for a particular callback must be performed in the shared "done" callback. Note that for the above example to work, the error object must carry enough information so that isKeyConflict() (whose implementation is not shown) can return true for insertion conflicts only. Otherwise, we have introduced a defect.

Only a linear chain of calls is supported. Branches and loops must be hand-rolled, or you have to use additional nested combinators. This doesn't matter for this example, but branches and loops aren't uncommon in interesting application code.

Now, goTo objects are not strictly superior to the alternatives in all situations. The pattern still has some overhead and boilerplate. For one or two levels of callbacks, you should probably just write in the naive nested callback style. If you have a linear callback chain of homogeneous type, or if you just don't care about statically typing the code, async.waterfall() has some advantages.

Plus, popping up a level, if you are writing lots of complex logic in your server, I'm not sure Node.js is even the right technology base. Languages where you don't have to write in continuation-passing style in the first place may be more pleasurable, terse, and straightforward. I mean, look: I've been reduced to programming with goto, the original harmful technology. By writing up this post, I'm trying to make the best of a bad situation, not leaping out of my bathtub crying eureka.

Anyway, caveats aside, I just thought I'd share this pattern in case anyone finds it useful. Yesterday I was chatting about Node.js with a friend and when I mentioned how I was handling callback hell, he seemed mildly surprised. I thought everybody was using some variant of this already, at least wherever they weren't using async. Apparently not.

p.s. The above pattern is, of course, not confined to Node.js. It could be used in any codebase written in CPS, in a language that has letrec or an equivalent construct. It's hard to think of another context where people intentionally write CPS by hand though.

Oh, that's very nice. Yes, TypeScript is syntactically cleaner and advancing way faster than the Closure Compiler, which is languishing as Google directs its attention to Dart and Angular. It also works better with CommonJS modules, which is the module style that most people use these days. This is a little like the problem Java has relative to C#.

Although now that I think about it, accurate typechecking for async.waterfall() in the example above requires more than just heterogeneous arrays. The typechecker has to somehow step through the callbacks in sequence and give the final parameter to each callback a type which is derived from (but not identical to!) the type of the next callback in the chain. So you might still want to use goTo objects in TypeScript.

BTW async.waterfall() is an example of why I think JavaScript needs a Turing-complete type system. async isn't even written in an unusually obscure style and there are tons of libraries like it where you basically have to surrender precise typechecking.

If the tuples passed to waterfall() are not of a constant length, the new tuple types won't help anyway.

Have you looked at promises? Your goTo solution looks vaguely like a promise-based solution. Libraries like q.js have a reasonable way of turning node APIs into promise-based code. TypeScript's generics also get you reasonable type-checking of promises, though in complex cases it's still too hard. Also, promises do have the issue of tricky reasoning about exceptional control flow.

Nearly all call sites of async.waterfall() use a constant-length array. You're right that heterogeneous tuples don't help with assigning a general type to async.waterfall() itself though.

I've looked a little at promises, although I have not used them much yet. Honestly, they look complicated as fuck, although it could be that I'm just getting dumber than I used to be. Maybe I'll write a followup post comparing goTo objects to Promises.